Carbon nanotubes are hollow cylinders of carbon atoms. Their appearance is that of rolled tubes of graphite, such that their walls are hexagonal carbon rings, and they are often formed in large bundles.
With the possession of metallic conductivity and semiconductor conductivity according to structures, carbon nanotubes are now top candidate to be applied to various technological fields such as, for example, electrodes of electrochemical storage devices (e.g., secondary cells or supercapacitors), electromagnetic shielding, field emission displays or gas sensors.